refactor: extract client Chat API to seperate file

[phodal]

Is your feature request related to a problem? Please describe. No

Describe the solution you'd like

in current design, all client use API in their components, like ChatRoot.tsx:

  async function createConversation() {
    const response = await fetch("/api/chatgpt/conversation", {
      method: "POST",
      body: JSON.stringify({
        action: "create_conversation",
        name: "Default name",
      } as RequestCreateConversation),
    });
   ...
  }

[CGQAQ]

I'll take it